1. Technical Field
This invention relates generally to laser angular rate sensors and more particularly concerns an improvement in apparatus which maintains the optical path length at an integer number of wavelengths of the laser light.
2. Description of the Prior Art
Path length control systems require a signal indicative of laser intensity. Typically this is provided by at least one photodiode and preamplifier.
In the prior art, shown in FIG. 1, a laser angular rate sensor having closed optical cavity ABC is shown with oppositely directed travelling waves 1 and 2 shown confined by reflective mirrors B and C and partially transmissive mirror A at the apices of the triangle. Two of the mirrors, at B and C, are movable in response to the voltage output of summing amplifier 10 so as to maintain the cavity tuned to form an oscillator at the optical lasing frequency. The mirror 3 at A is partially transmissive to allow a processing of light from beams 1 and 2 by combiner prism 5 producing a fringe pattern on dual photodiode 20. Photocurrents produced in the dual photodiode 20 are amplified by dual fringe detector preamplifier 12, the outputs of which are processed to produce incremental angular output pulses in response to sensor rotation. Light from beam 1 is internally partially reflected at two surfaces 4 and 6 of mirror 3 and transmitted to photodiode 22, the photocurrent of which is amplified by preamp 13 and further processed to serve as a measure of cavity tuning.
Referring to FIG. 2, a tuned cavity produces maximum laser intensity, shown at point a on the intensity-versus-path length curve. A small sinusoidal modulation b of the path length produces no corresponding intensity modulation at that frequency due to the flatness of the curve at point a. The known approach recognizes that the same modulation c at point d on the curve produces an output e bearing the same phase as c, due to the positive slope of the curve, at point d. Were the modulation at point f, the output phase would be reversed (not shown). The magnitude and phase of the intensity modulation is used by the path length control loop as an error indication to cause automatic respositioning of the movable mirror 7 and/or 9 of FIG. 1 to achieve an optimum operation at point a of the gain curve of FIG. 2.
Referring again to prior art FIG. 1, oscillator 11 produces a sinusoidal voltage applied to amplifier 10 which in turn vibrates mirror 7 and/or 9. That voltage is also applied to demodulator 14 as a phase reference in the phase-sensitive demodulation of the signal at the output of preamplifier 13. Filter 15 in some implementations of path length control (PLC) loops is often not used.
One problem with the prior art approach lies with the small signal amplitude of the photocurrent in photodiode 22 requiring substantial gain in preamp 13. The small value of photocurrent results from the low level of light intensity impinging on photodiode 22. This light is but a fraction of that transmitted through mirror 3 from beam 1. At interface 4 between mirror 3 and prism 5, the major portion of the light is transmitted into prism 5 where after three internal reflections at the prism surfaces shown, it is transmitted onto photodiode 20 along with light directly transmitted from beam 2. The light reflected at interface 4 loses further intensity upon reflection at point 6 and transmission through point 8. As a result of the low intensity, the signal-to-noise ratio of the signal impinging on photodiode 22 is degraded as is the performance of the path length control function.
A second problem involves compromising the design of the optics to produce transmitted beams for both fringe detector and path length control functions. To produce a beam to illuminate photodiode 22, the interface at 4 must intentionally be partially reflective, thus subtracting from light transmitted into prism 4 and ultimately reducing the light flux available to illuminate fringe detector photodiode 20.